Pediatric Cerebral Aneurysms

Rupture of saccular cerebral aneurysms is the most common cause of atraumatic SAH in children. Aneurysms are localized, abnormal dilations of blood vessels, usually arising from arteries. True aneurysms involve all vessel-wall layers, which are thinned but generally complete. The blood within true aneurysms participates in the general circulation. Atherosclerotic, syphilitic and congenital aneurysms are considered true aneurysms. A false aneurysm, or pseudoaneurysm, is an extravascular hematoma in communication with the intravascular space that has been confined by adventitia or "walled off" by fibrous tissue, a process often incorporating the entire circumference of the vessel. False aneurysms are commonly found at leaks from the junction of a synthetic graft with a natural artery, as well as in moyamoya disease.

It is believed that aneurysms result from congenital or genetic predisposition in conjunction with acquired factors such as hypertension, diabetes and cigarette smoking. Acquired factors are typically absent in children, which suggests that the patho-physiology is different than in adults. In addition, pediatric aneurysms differ from adult aneurysms in location, size and clinical presentation, thus making the pediat-ric aneurysm a unique entity for the neurosurgeon with respect to surgical considerations and overall outcome.

Epidemiology Incidence

Although presumed to be congenital in origin, aneurysms are infrequently encountered in the pediatric population. Only 12% of all aneurysms occur in children. Most occur after 10 years of age, with 80% of patients presenting in the second decade of life. The age of presentation for aneurysm is bimodally distributed, with peaks occurring at birth to age 6 and then again at 8 years of age through adolescence. Pediatric aneurysms occur more often in males, with a male to female ratio of 1.3:1. Under 5 years of age, the male to female ratio rises to 4. In contrast, the gender predilection is opposite in the adult population, where the female to male ratio is 1.6:1.

Other Features

Both adult and pediatric aneurysms typically occur at bifurcation points in the cerebral vasculature, but the vessels involved differ significantly. Pediatric aneurysms most frequently occur in the middle cerebral or vertebrobasilar arteries, with vertebrobasilar circulation aneurysms most often seen during the ages of birth through age 2. Posterior circulation aneurysms are 3 times more common in the pediatric population. Aneurysms in the MCA distribution tend to occur at more peripheral sites in children than in adults. These distal lesions are associated with an increased incidence of intracranial hematoma. Thirty percent occur at the ICA bifurcation, however these aneurysms commonly present during adolescence.

Multiple aneurysms are uncommon in children, occurring only in 4% to 5% of patients. Multiple aneurysms in children more often are encountered with other conditions, such as moyamoya disease, AVM, fibromuscular dysplasia, sickle cell disease and previous cranial irradiation. Saccular aneurysms diameters greater than 25 mm in diameter are considered giant aneurysms and constitute a larger proportion of aneurysms seen in children as compared to adults. Giant aneurysms can present with symptoms of a space-occupying lesion. Thirty percent of aneurysms in those under 5 years of age are giant, and are 6 times more likely to be located in the posterior circulation as compared to adults. Again, gender predilection is opposite than that observed in adults: 60% of adult giant aneurysms occur in females, whereas 62% of pediatric giant aneurysms occur in males.

Pathology Pathogenesis

The pathogenesis of idiopathic saccular aneurysms is controversial. Aneurysms arise from sites containing defects of the muscularis layer and the internal elastic membrane of the tunica media. It is believed that medial defects are usually congenital. Unsupported elastic tissue from a vessel has been shown to withstand pressures up to 600 mm Hg, thus making the notion of elastic elements herniating through congenital defects of the muscularis unlikely. In addition, medial defects are commonly observed in coronary and mesenteric vessels, sites where aneurysms are exceedingly rare. Of note is that intracranial vessels have only one elastic lamina,

Table 3. Diseases associated with aneurysmal formation

Aortic coarctation Arteriovenous malformation Polycystic kidney disease Tuberous sclerosis Fibromuscular dysplasia Ehlers-Danlos syndrome Marfan's syndrome a relatively thin tunica media, and receive relatively little structural support from adjacent tissue. The vasa vasorum is absent in the smaller vessels.

It is believed that most aneurysm arise at sites where a congenital defect of the muscularis is present along with injury to the internal elastic membrane. In adults, atheromatous change and hypertension are acquired factors that contribute to aneu-rysm formation. In children, hypertension and atheromatous changes are rarely observed. Hypertension, however, can coexist in diseases such as aortic coarctation and polycystic kidney disease, which are associated with intracranial aneurysms (Table 3). Finally, aneurysm formation can complicate closed-head injury. Traumatic aneu-rysms have disrupted wall layers, except for the adventitia, and occur in typical locations such as the anterior cerebral artery adjacent to the falx, the MCA along the sphenoid ridge, and the posterior cerebellar artery along the tentorium.

Mycotic Aneurysms

Mycotic aneurysms are more appropriately termed "infectious aneurysms," since most are caused by bacterial rather than fungal infections. Infectious aneurysms most commonly occur due to septic emboli, such as in bacterial endocarditis. The course of aneurysmal formation is alarmingly short; hemorrhage can occur within two days of the embolic event. The most commonly encountered organisms in pediatric intracra-nial infectious aneurysms are a-Streptococcus, Staphylococcus, Pseudomonas and Haemophilus species. Infectious aneurysms are classified into three primary forms:

1. Embolization from bacterial endocarditis (most common).

2. Direct extension from site of infection such as meningitis, sinusitis or osteomyelitis.

3. Idiopathic infectious aneurysm diagnosed by histology with no other inflammatory lesion identified.

Clinical Features

In both children and adults, SAH is the most common initial presentation and presents with a typical clinical picture: a sudden onset of a severe headache, associated with vomiting, irritability and seizures. The patient's level of consciousness often deteriorates with a rapid progression to coma or death. Other features include retinal hemorrhage, a bulging fontanelle, hydrocephalus, extremity paresis, cranial nerve palsies and fever. Ten to 15 percent of children have a less severe initial headache, termed a sentinel bleed, prior to the presenting symptoms. Focal

neurological signs due to mass effect from the expanding aneurysm can be transient and are often missed in children.

Symptoms from mass effect are more often due to intracranial hypertension following aneurysm rupture. One-third of children with intracerebral aneurysms have signs of intracranial hypertension, as a result of either aneurysm size or hydroceph-alus. Hypertensive symptoms include nausea, vomiting, lethargy, papilledema, a bulging fontanelle and focal neurological signs such as third- and sixth-nerve palsies.

Evaluation

Asymptomatic Children

Asymptomatic children with high-risk factors should have a screening MRA or cerebral angiography. Risk factors in this population include:

1. Strong family history of aneurysm (2 or more family members known to have intracranial aneurysm)

2. Aortic coarctation

3. Polycystic kidney disease

4. Fibromuscular dysplasia

5. Ehlers-Danlos syndrome

Symptomatic Children

A noncontrast CT scan or MRI should be performed to evaluate for subarach-noid blood. These tests are most sensitive if performed within 5 days of hemorrhage. CT also is useful to evaluate for hydrocephalus, cerebral swelling and intraparenchymal hematoma. Lumbar puncture (LP) should be done only if the CT scan does not reveal SAH. Raised IPC from a mass lesion such as intracranial hematoma or hydrocephalus is a contraindication for an LP. If a CT scan, MRI scan, or CSF analysis demonstrate evidence of a SAH, a standard, 4-vessel cerebral angio-gram should be obtained. Although there is some controversy regarding the utility of repeat angiography in cases where the initial study is negative, a complete angio-gram with both intracranial and extracranial vessels visualized usually does not need to be repeated. A treatment outline is listed in Table 4.

Surgical Treatment of Aneurysmal SAH

Timing of Surgery

The risk of rebleeding is highest in the first 24 hours after initial hemorrhage. For this reason, surgical treatment should be scheduled without delay. In very ill children (high grade), surgery may need to be postponed 7 to 14 days to allow their condition to stabilize. The risk of rebleeding is probably less in a child than an adult, although accurate statistical data are not available.

Surgical Techniques

Direct surgical clipping of the aneurysm neck is the first choice of treatment for most patients. Other standard techniques utilized include:

1. Trapping. Occluding the parent vessel above and below the aneurysm; usually only possible if collateral flow is present above the aneurysm.

Table 4. General treatment protocol for patients with subarachnoid or intracerebral hemorrhage

Maintain normotension and normovolemia

Seizure prophylaxis Raised ICP

Vasospasm (for aneurysmal SAH)

Electrolytes

Strictly avoid hypertension

Monitor volume status carefully (arterial catheter, CVP line andd/or Swan-Ganz catheter)

Antiepileptic agents, phenytoin and phenobarbital are most commonly used

Ventriculostomy if there are signs of hydrocephalus

Avoid valsalva by utilizing cough suppressants, stool softeners, restful environment and antiemetics

Nimodipine to reduce risk of vasospasm, although benefits of nimodipine in pediatric population is unclear

Hyponatremia may occur due to SIADH (normovolemia) or cerebral salt wasting (hypovolemia); electrolytes and osmolality should be followed

2. Wrapping. A small amount of gauze is wrapped around the aneurysm causing an inflammatory reaction that is expected to isolate the sac.

3. Aneurysmectomy. For giant aneurysms, where a large sac has to be removed and the neck secured with a clip or suture.

4. Extracranial-intracranial bypass. Used when an aneurysm can only be trapped and distal flow must be reconstituted using a vessel or synthetic bypass.

5. Endovascular management is the treatment of choice for many posterior circulation aneurysms and those with an advantageous configuration elsewhere.

Outcome

The mortality rate for intracranial aneurysm after the first SAH ranges from 11% to 20% in children, compared to 20% to 30% in adults. Rates for rebleeding also are lower in children, ranging from 7% to 13%, while in adults the rate of rebleeding is approximately 20%. In children, the mortality rate following rebleeding is 25%.

Suggested Readings

1. Albright AL, Pollack IF, Adelson PD, eds. Principles and Practice of Pediatric Neurosurgery. New York: Thieme, 1999.

2. Barrow DL ed. Intracranial vascular malformations. In: Neurosurgical Topics. Park Ridge: American Association of Neurological Surgeons, 1990

3. Hamilton MG, Herman JM, Khazata MH. Aneurysms of the vein of galen. In: Neurological Surgery. Philadelphia: WB Saunders, 1996.

4. Raimondi AJ, Choux M, DiRocco CD, eds. Cerebrovascular Diseases in Children. New York: Springer Verlag, 1991.

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